Diazonium salt compounds, having extremely high chemical activity, react with compounds such as phenol derivatives and compounds having an active methylene group (generally called couplers) to form azo dyes. Since diazonium salt compounds also are light sensitive and lose their activity upon irradiation with light, they are commonly utilized in photo-recording materials (see "Shashin K ogaku No Kiso (Fundamental Photographic Technology)--Hi-ginen Shashin Hen (a volume of Non-Silver Halide Photography)," edited by Photographic Science and Technology Society of Japan, Corona Sha (1982) pp.89-117, pp.182-201). Due to these properties, diazonium salt compounds also have recently been applied to heat-sensitive recording materials. A photofixation-type heat-sensitive recording material has been proposed in which a diazonium salt compound is thermally reacted with a coupler to form an image, which image is then fixed by irradiation with light (e.g., K oji Sat o et at., Gaz o Denshi Gakkai-shi (Journal of Image Electronics Society), Vol.11, No.4 (1982) pp.290-296). However, prior art recording materials employing diazonium salt compounds have a short shelf life because diazonium salt compounds gradually undergo pyrolysis, even in the dark, due to their extremely high activity. To overcome this drawback, a technique of microencapsulating a diazonium salt compound to thereby isolate the same from substances which accelerate pyrolysis, such as water or a base, has been proposed (Toshimasa Usami et al., Denshi Shashin Gakkai-shi (Journal of Electrophotographic Society), Vol.26, No.2 (1987) pp.115-125).
In a generally employed process for microencapsulation of a diazonium salt compound, the diazonium salt is dissolved in an organic solvent (oil phase), and the solution is added to and emulsified into an aqueous solution of a water-soluble polymer (aqueous phase). This emulsification is conducted in the presence of a monomer or prepolymer which is to be a wall material of the capsule. The monomer or prepolymer is added beforehand to either the organic solvent phase or the aqueous phase. As a result, polymer walls form at the interface between the organic solvent phase and the aqueous phase to produce microcapsules. Such methods are described in detail, e.g., in Asashi Kond o, "Microcapsule", Nikkan Kogyo Shinbun-sha (1970) and Tamotsu Kond o et al., "Microcapsule", Sanky o Shuppan (1977). For forming microcapsule walls, various materials can be used, such as gelatin, alginic acid salts, cellulose derivatives, urea resins, urethane resins, melamine resins, and nylon resins. Of these, urea resins and urethane resins are preferred from the standpoint of designing a heat-sensitive recording material. Namely, these resins have a glass transition point in the range of from room temperature (about 20-30.degree. C.) to one hundred and several tens of .degree. C., so that microcapsule walls made of such materials are responsive to heat.
Diazonium salt compounds have high chemical activity and tend to readily undergo pyrolysis or hydrolysis. On the other hand, it is known that by microencapsulation, these compounds are isolated from highly reactive substances. As a result, the microencapsulated diazonium salt compounds are less sensitive to such highly reactive substances and therefore exhibit improved stability.
In the case of microcapsules having urea resin or urethane resin walls, a generally employed process for producing the microcapsules comprises first dissolving a diazonium salt in an organic solvent, adding an isocyanate compound thereto, emulsifying the resulting organic solvent solution into an aqueous solution of a water-soluble polymer, and then polymerizing the isocyanate compound by either adding a polymerization catalyst to the aqueous phase or elevating the temperature of the emulsion to thereby form capsule walls. This technique has succeeded in prolonging the shelf lives of photo- and heat-sensitive recording materials employing microcapsules containing a diazonium salt compound to about a few years, which recording materials have conventionally had a shelf life of less than one year. However, such photo- and heat-sensitive recording materials employing microcapsules containing a diazonium salt compound are defective in that background staining becomes severer with the lapse of time. In addition, the microencapsulated diazonium salt produced by the prior art technique still have insufficient stability.
As a result of intensive studies made by the present inventor, it has been found that the stability of a microencapsulated diazonium salt compound and the increased background staining with time in photo- and heat-sensitive recording materials employing microcapsules containing a diazonium salt compound vary depending on the additives used in the microencapsulation process, and that the addition of a specific additive imparts extremely good stability to the microencapsulated diazonium salt compound to produce a recording material which undergoes little increase in background staining with the lapse of time. The present invention has been achieved based on these findings. The effects of the present invention are pronounced in the case where the material of the capsule walls is a polyurea or a polyurethane (i.e., where a polyfunctional isocyanate is used as a material for the microcapsule walls).